Support for multiple catch clauses (SpiderMonkey-specific)

Spidermonkey AST can contain multtiple catch clauses. This diff supports them.
Here's the sample js which fails on current escodegen@1.0.1 and runs successfully with this diff:

```javascript
try {
  smth();
} catch (ex if ex instanceof TypeError) {
  console.log('TypeError');
} catch (ex if ex instanceof ReferenceError) {
  console.log('ReferenceError');
}
```